'Virgin Earth' Rock Survived Massive Cosmic Collision

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Billions of years ago, our baby planet was smashed by another planetary body, turning it into a burning ball of molten rock. But according isotopes recovered from deep inside the Earth’s mantle, some of the pre-impact material persists to this day, possibly proving that some of our planet survived the cosmic collision intact.

During that epoch of our solar system’s evolution, planetary collisions were commonplace and it is thought that a hypothetical Mars-sized body, nicknamed “Theia,” hit Earth in a cataclysmic collision some 4.5 billion years ago. The energies released during impact would have totally transformed our planet, obliterating its surface and melting its rocky mantle.

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But the extent of this planetary transformation isn’t well understood. Was Earth completed melted? Or have some pockets of material of a primordial Earth persisted to modern day, proving that not all terrestrial material was affected by the Earth-Theia encounter?

“The energy released by the impact between the Earth and Theia would have been huge, certainly enough to melt the whole planet,” said geochemist Sujoy Mukhopadhyay of Harvard University and lead scientists of this research. “But we believe that the impact energy was not evenly distributed throughout the ancient Earth. This means that a major part of the impacted hemisphere would probably have been completely vaporized, but the opposite hemisphere would have been partly shielded, and would not have undergone complete melting.”

Mukhopadhyay is presenting his team’s work at the Goldschmidt geochemistry conference in Sacramento, Calif., this week.

The research focuses on the comparison of noble gas isotopes in the deep mantle compared with the shallow mantle. The Earth’s mantle is a silicate rocky shell that extends from the crust to as deep as 1,800 miles (2,900 kilometers) to the Earth’s molten outer core. The mantle is differentiated into different mineral layers that provide information about our planet’s ancient geochemical past.

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The researchers analyzed ratios of isotopes of Helium (3He) and Neon (22Ne) and found that the ratio was significantly higher in the shallow mantle than it was in the deep mantle. “This implies that the last giant impact did not completely mix the mantle and there was not a whole mantle magma ocean,” said Mukhopadhyay in a press release.

In addition, they analyzed the 129-Xenon to 130-Xenon ratio from material transported from the deep mantle to the surface by mantle plumes — again, the ratio was significantly lower in material from the lower mantle when compared to ratios found at the surface. The Xenon ratio is interesting as 129-Xenon is produced by the radioactive decay of 129-Iodine, putting a definite ‘time stamp’ on the transported deep mantle material to within the first 100 million years of Earth’s early history.

“The geochemistry indicates that there are differences between the noble gas isotope ratios in different parts of the Earth, and these need to be explained,” said Mukhopadhyay. “The idea that a very disruptive collision of the Earth with another planet-sized body, the biggest event in Earth’s geological history, did not completely melt and homogenize the Earth challenges some of our notions on planet formation and the energetics of giant impacts.

“If the theory is proven correct, then we may be seeing echoes of the ancient Earth, from a time before the collision.”

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In other research published in the journal Science last week, isotopic analysis of elements inside moon rock (rock recovered by the Apollo missions from the lunar surface and moon meteorites recovered on Earth) revealed the chemical signature for Theia and geologists have been able to deduce that around 50 percent of the moon is likely composed of material originating from the interplanetary impactor.

There is little doubt that the impact of Theia was was a global event, turning our planet into a burning blob of magma and even spawning the formation of our moon, but it appears that the picture is a little more complex when trying to work out how much of our planet was transformed by the impact.

Deep down in the Earth’s mantle, material from a pre-impact world appears to be lurking, providing us with an intriguing geological time capsule of a very alien ‘virgin’ Earth.